1) Field of the Invention
The present invention relates to an improvement of non-aqueous electrolyte secondary cells, which improvement is intended to improve discharge capacity and cycle characteristics.
2) Description of the Related Art
In recent years, there has been a rapid reduction in the size and weight of mobile information terminals such as mobile phones, notebook personal computers, and PDAs. High capacity and high energy density are required of cells and batteries serving as the driving power sources of such terminals. Non-aqueous electrolyte secondary cells represented by lithium ion secondary cells have high energy density and high capacity and as such are useful as the driving power sources of the mobile information terminals.
As a positive electrode active material of such non-aqueous electrolyte secondary cells, lithium cobalt compound oxide (LiCoO2) is used for its high capacity and excellent charge-discharge characteristics.
As the progress of higher functionality of mobile information terminals, higher capacity is required of the cells. In view of this, such a positive electrode active material is used that is charged to a higher potential than usual in an attempt to increase the use efficiency of the positive electrode active material.
However, if the lithium cobalt compound oxide is charged to a potential higher than 4.3V with respect to lithium, the stability of the compound is significantly reduced, presenting the problem of greatly deteriorating cycle characteristics.
In order to solve this problem, such a technique is proposed that by adding a different metal such as zirconium and magnesium in the lithium cobalt compound oxide, the stability of the compound is increased at high potential.
However, even with this technique, the thermal stability at high potential is not sufficient. In addition, in cells according to this technique, the electrolytic solution is decomposed through charge-discharge cycling, presenting the problem of deteriorating cycle characteristics.
Techniques related to the non-aqueous electrolyte secondary cell are proposed in patent documents 1 to 4, and the outlines are as follows.
Patent Document 1: Japanese Patent Application Publication No. 2002-313419 (paragraphs 0004 to 0007).
Patent Document 2: Japanese Patent Application Publication No. 2002-75448 (paragraphs 0008 to 0029).
Patent Document 3: Japanese Patent Application Publication No. 2003-308842 (claims, paragraphs 0009 to 0012).
Patent Document 4: Japanese Patent Application Publication No. 2004-134366 (paragraphs 0007 to 0009).
(i) Patent document 1 proposes a technique that uses a solvent containing at least, as the solvent components, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate at a volume ratio of 25 to 40 vol %, 25 to 60 vol %, and 10 to 40 vol %, respectively.
This technique is for the purpose of obtaining a cell that has high capacity, does not suffer swelling caused by gas generation, and has good low-temperature characteristics. However, this technique does not take into consideration the use of the positive electrode active material at high potential, and further improvement is required in this respect.
(ii) Patent document 2 proposes a technique that uses, as the non-aqueous solvent, a mixture solvent of ethylene carbonate and a low-boiling-point solvent excluding dimethoxyethane.
This technique is for the purpose of obtaining a lithium secondary cell that has high cell capacity, has low self-discharge rate, excels in cycle characteristics, and has high charge-discharge efficiency. However, this technique does not take into consideration the use of the positive electrode active material at high potential, and further improvement is required in this respect.
(iii) Patent document 3 proposes the following technique. The positive electrode used here contains, as a positive electrode active material, lithium-manganese-nickel compound oxide that is contained in the positive electrode mixture and generates approximately 5 V with respect to lithium in a fully-charged state. The negative electrode used here uses a negative electrode active material that can intercalate and deintercalate lithium ions during charging and discharging. The non-aqueous electrolytic solution used here is such that lithium salt is dissolved in a solvent that contains ethylene carbonate or propylene carbonate. By immersing such positive electrode and negative electrode in this non-aqueous electrolytic solution, lithium phosphate is contained in the positive electrode mixture.
According to this technique, by having lithium phosphate contained in the positive electrode mixture, the reaction product that results from the reaction of the lithium phosphate and non-aqueous electrolytic solution protects an active portion on the surface of the positive electrode active material. This inhibits the decomposition of the non-aqueous electrolytic solution, making it possible to improve the charge-discharge efficiency. However, with this technique, since lithium-manganese-nickel compound oxide (LiMn2-xNixO4) of spinel structure is used (paragraph 0012), this compound oxide has only 1 mole of the lithium per 2 moles of the manganese and nickel combined. Since the amount of the lithium contributing to the charge-discharge reaction is thus small, high cell capacity cannot be secured sufficiently.
(iv) Patent document 4 proposes the following technique. As the non-aqueous electrolytic solution, such a non-aqueous solvent is used that ethylene carbonate, dimethyl carbonate, and diethyl carbonate meet the formulas y=z, 20<x<30, and x≦2y/3+10, where x, y, and z represent the mixture ratios by volume percent for the ethylene carbonate, dimethyl carbonate, and diethyl carbonate, respectively, with respect to the non-aqueous electrolytic solution.
This technique is for the purpose of obtaining a lithium-ion secondary cell that excels in high-efficiency charge-discharge characteristics at low temperature. However, this technique does not take into consideration the use of the positive electrode active material at high potential.